Superconductor digital integrated circuits present multilayered structures consisting of superconducting Josephson junctions connected to resistors, inductors, capacitors, superconducting transmission lines and other circuit elements by superconducting layers separated by insulating layers. In the most common embodiments, Josephson junctions (JJ) present Nb/Al/AlOx/Nb or Nb/Al/AlOx/Al/Nb tunnel junctions, superconducting layers are Nb layers, and insulating layers are SiO2 layers. Superconducting layers are placed both above and below the layer of Josephson junctions, and the number of layers depends on the fabrication process and circuit design. Fabrication processes containing up to 10 superconducting (Nb) layers have been described. One embodiment of the prior art is a standard Hypres IC process (Hypres Inc., Elmsford N.Y.), as shown in FIG. 1.
Superconducting Nb layers can be replaced by Nb-based alloys or compounds such as Nb—Zr, Nb—Ti, niobium nitride NbNx, niobium-titanium nitride NbTiNx, niobium carbonitride NbCxNy and similar. The tunnel barrier material of Josephson junctions, in addition to aluminum oxide AlOx, can also be aluminum nitride AlNx, magnesium oxide MgOx, and other dielectrics. In some implementations, the tunnel barrier is replaced by a poorly conducting metal or a semiconductor layer such as variously doped Si, Ge, GaN, silicides of transition metals, etc. The main requirements for superconducting materials and the barrier material of Josephson junctions is that the manufactured Josephson junctions in superconductor integrated circuits should be uniform, reproducible, and stable. It is noted that an aluminum oxide tunnel barrier is typically formed such that the aluminum layer is incompletely oxidized, leaving a thin aluminum layer, since the quality of a niobium oxide layer formed under the oxidizing conditions suitable for aluminum oxidation is poor. The Josephson junction “trilayer” may therefore actually be a quad layer device.
Performance and operation margins of Rapid Single Flux Quanta (RSFQ) circuits are very sensitive to circuit parameter spreads, especially to variations in the values of critical currents of Josephson junctions comprising the circuits. Therefore, an important requirement of high-yield manufacturing technology for superconducting digital circuits is to reproducibly deliver Josephson junctions having minimal deviations of critical currents, Ic, from the Ics required by the circuit design and optimization. The most advanced fabrication technology capable of superconducting very large scale integrated (VLSI) circuits has been Nb-based technology that utilizes Nb/Al/AlOx/Nb Josephson junctions and multiple layers of Nb for circuit inductors, interconnects, and signal routing.
It has been found recently that in Nb circuits, in addition to small and random variations of critical currents of JJs, there can exist large and systematic deviations of critical currents of JJs from the expected (design) values [1]-[4]. The former are caused by statistical fluctuations in the junction area and tunnel barrier transparency, and can be characterized by a standard deviation, σIc. The latter means that the value of Ic deviation in a specific junction or a group of junctions, though potentially varying from run to run, many times exceeds σIc, so the probability of this happening as a result of random fluctuations is statistically negligible. For instance, the Jc in a JJ may depend on how the junction is connected to other circuit layers and on the area and shape of the contacting layers [1]-[2], whether the junction base electrode (BE) or counter electrode (CE) makes contact to Nb ground plane layer M0 [1]-[2], on the distance between the junction and the contact hole to other layers, and on the number of contact holes [3]. The critical current density and the gap voltage may increase over time in junctions stored at room temperatures if one or both of the junction electrodes are connected by Nb wire to Ti/Au or Ti/Pd/Au contact pads or just covered by a layer of Ti [4]. The effect is larger if CE is connected to a Ti-coated layer than if BE is connected.
The dependences of Josephson junction properties on the junction's environment and circuit patterns have been explained as resulting from hydrogen contamination of Nb circuit layers during wafer processing, with its subsequent migration towards or away from the AlOx tunnel barrier during the manufacturing cycle and later on upon its completion [3]-[6]. Long-term changes in Ic were first found in Nb circuits with Pd coating [5],[6] and were suggested to be caused by hydrogen absorption and desorption.
It is well known that bulk Nb and Nb films can dissolve large amounts of hydrogen at room temperature, up to cH˜50 atomic percent, where cH is the hydrogen content (H/Nb ratio). Hydrogen is the most mobile impurity [7]. Its diffusion coefficient in Nb at 300 K is D˜10−5 cm2/s; and diffusion activation energy is 0.106 eV, the lowest of all impurities (next is oxygen with the activation energy ˜1 eV [8]).
Dissolved hydrogen changes many physical properties of Nb, e.g., it increases the lattice constant, resistivity, etc. (for a review, see [9]). Most importantly, it increases the work function of hydrogen-contaminated niobium, Nb(H), with respect to the clean Nb [10],[11]. As a result, the average height of the tunnel barrier in Nb/Al/AlOx/Nb junctions and the barrier asymmetry become dependent on the cH in the CE near the tunnel barrier, as was proposed in [4] (see also [6]). As was also suggested, the Nb base electrode in real Josephson junctions is coated by a thin Al layer which is only partially consumed by the oxidation forming an AlOx tunnel barrier. As a result, the tunnel barrier in Nb/Al/AlOx/Nb junctions is asymmetric (trapezoidal). The barrier height on the BE side, φBE, is determined by the work function of Al, and the barrier height on CE side, φCE, is determined by the work function of Nb, resulting in φCE>φBE. [12]-[14]. Therefore, the presence of hydrogen in the Nb base electrode has no effect on the tunnel barrier height and hence has much less effect on the critical current density of Josephson junctions than hydrogen dissolved in the Nb counter electrode, as was explained in [4] (see also [6] and [15]).
In addition to a reversible effect on the Nb work function, it is possible that hydrogen can chemically react with the AlOx barrier and cause irreversible changes to its properties, e.g., create states with high transmission probability. Because diffusion of dissolved hydrogen in an integrated circuit occurs on a complex network of Nb wires interconnecting multiple junctions, resistors, and inductors, all with different diffusion coefficients and cross sections, complex concentration distributions may appear and depend on details of a particular circuit design. This can affect different circuits in a different, though always negative, and quite reproducible manner.
Theoretically, H2 concentration in air at atmospheric pressure is sufficient to saturate Nb with hydrogen at room temperature. The Nb surface presents a potential barrier for H2 molecules and the native oxide on the surface works as a diffusion barrier, and both prevent hydrogen absorption. However, in all situations when the surface oxide is removed, hydrogen can easily dissolve in Nb. Hydrogen contamination can also occur because of the reaction with water molecules (in water, aqueous solutions, and moist air) Nb+H2O→NbO+H2 and due to a charge transfer process on a clean Nb surface H2O+e−→OH−+H forming highly active atomic hydrogen which easily dissolves in Nb. Processes such as chemical etching, reactive ion (plasma) etching, chemical mechanical polishing (CMP), ion milling of Nb remove surface oxide and, hence, can produce hydrogen contamination. During Nb film deposition by sputtering or other methods, hydrogen contamination can easily occur if there is sufficient residual hydrogen or water pressure in the vacuum chamber. After the deposition, hydrogen contamination is possible upon removing the deposited Nb film from the vacuum chamber because the clean surface of a freshly deposited film can readily react with air moisture.
The spatial variation (spread) of the physical parameters of Josephson junctions over the chip area and wafer area, such as the Josephson critical current density Jc, critical current of the junctions Ic, normal state conductance Gn, superconducting energy gap, subgap conductance and leakage current, and their physical dimensions can thus be non-uniform. The manufacturing yield of superconducting integrated circuits decreases strongly with increasing the on-chip and on-wafer spreads of parameters of Josephson junctions.
Reproducibility characterizes the ability of the manufacturing process to reproduce the same parameters of Josephson junctions from wafer to wafer and run to run. High-yield manufacturing requires high reproducibility of parameters of Josephson junctions.
Long-term stability of parameters of Josephson junctions is required for implementations of superconductor circuits in electronic devices and systems as well as their stability against thermal cycling between the temperature of their operation (usually around 4.2 K) and the temperature of their storage at room or slightly elevated temperatures, typically in the range from 290 K to 350 K.
One of the most important factors determining the manufacturing yield of superconductor integrated circuits based on Josephson junctions is deviations of the critical current of manufactured Josephson junctions from the proper (target) values dictated by the circuit design. Usually, deviations of Ic of just a few junctions by more than about ±10% from their target values can make digital circuits based on Rapid Single Flux Quantum (RSFQ) logic with about 104 of Josephson junctions completely non-operational. Yet smaller deviations become critical as the number of Josephson junctions in the circuit increases further. The critical current of a Josephson tunnel junction in general depends on the superconducting energy gaps in the junction electrodes and on the transparency of the tunnel barrier. The barrier transparency in turn depends on the tunnel barrier height, thickness, and the amount defects in the barrier which locally alter the average transparency. There are two types of deviations—small random deviations and systematic deviations.
Nb, Ti, Pd, and many other transition metals and their alloys which are used in manufacturing of superconductor integrated circuits can dissolve (absorb) many gaseous impurities, especially hydrogen, during many steps of wafer processing such as thin film deposition, dry etching using reactive plasmas, chemical etching, chemical mechanical polishing, ion milling, etc. Since different layers in the multilayered structure of the integrated circuit may undergo a different number of processing steps, the concentration of absorbed impurities (e.g., hydrogen) may be different in different layers. Different superconducting layers, Josephson junctions, resistors, and inductors in integrated circuits are connected to each other using contact holes, vias, and plugs, to form an integrated circuit. These connections create a complicated network of passages along which diffusion of impurities (e.g., of hydrogen) may occur during the wafer fabrication and later during the storage of manufactured circuits. Hydrogen is known to have a high diffusion coefficient (mobility) in Nb, Ta, Ti, Pd and many other transition metals and alloys at room temperature typical of wafer storage and slightly elevated temperatures which may be used for some of the wafer manufacturing steps. The wafer temperature during plasma enhanced chemical vapor deposition of silicon dioxide (SiO2) can be as high as 500 K.
As a result of interlayer diffusion, a distribution of impurities appears around electrodes of Josephson junctions, which depend on the circuit patterns. The distribution is dependent on how particular junctions are connected to other metal layers and circuit elements, on the distance between junctions and contacts between different layers, on the area of metal layers to which a particular junction is connected to, and so on. Since the critical current of Josephson junctions depends on the concentration of impurities in the electrodes, the critical current of Josephson junctions in integrated circuits also become dependent on the circuit pattern, the way a particular junction is connected (wired) to other circuit elements, the area and type of metal layers to which the connection is made, etc. For instance, it has been shown that the critical current of Josephson junctions connected by their base electrode to a ground plane Nb layer can be substantially larger that the critical current of otherwise identical junctions which are not connected directly to the ground plane. The difference was found to depend on the area of the ground plane and on the distance between the junction and the contact hole to the ground plane. It was also found that junctions which are connected by superconducting (Nb) wires to Ti/Pd/Au or Ti/Au metallization on the input/output contact pads also have higher critical currents that the otherwise identical junctions but not connected directly to the same Ti/Pd/Au or Ti/Au contact pads. (In this case connections can be made through additional Josephson junctions in series with the junction under investigation or by interrupting a direct connection by Nb wire with some other material, e.g. Mo.)
A significant drawback of the existing practice is that the critical current of Josephson junctions in superconductor integrated circuits may depend on the circuit patterns, on the type of metal layer to which a particular junction is connected, on the area of circuit element the connection is made to (e.g., on the area of the circuit ground plane). The critical current of a Josephson junction may also depend on which electrode (base electrode or counter electrode of the junction) makes a connection to the circuit ground plane. It may also depend on the presence of contact holes (vias, plugs) to other circuit layers in the proximity of the junction. It may also depend on how the junction is connected to input/output contact pads, which are usually covered by Ti/Pd/Au or T/Au metallization, e.g. on the length of the connecting wire or the type of metal or if the wiring is interrupted by a resistor on not.
All the mentioned above and similar pattern-dependent effects on properties of Nb-based Josephson junctions significantly reduce the manufacturing yield of superconductor integrated circuits. In general, any deviations of critical current of Josephson junctions in digital integrated circuits from the design values reduce the margin of operation and the maximum clock frequency, and hence the circuit yield.